Hybrid type forming section for a paper making machine

ABSTRACT

A twin fabric hybrid forming section for paper making machine is described in which: the pitch of the fabric support elements decreases progressively in the machine direction; the level of vacuum applied to the forming fabrics through the dewatering boxes increases in the machine direction; the two forming fabrics together with the stock sandwiched between them traverse at least four separate and distinct vacuum zones within the forming section as they proceed in the machine direction; the level of vacuum applied to the last of the at least four separate and distinct vacuum zones is higher than the level of vacuum applied to the first of the separate and distinct vacuum zones; the level of vacuum applied to the at least four separate and distinct vacuum zones follows a preselected profile; and the dewatering boxes carrying the fabric support elements are arranged so that the fabric support elements are located in an alternating sequence on the machine sides of both of the forming fabrics.

BACKGROUND

This invention relates to a twin fabric hybrid forming section for usein a paper making machine. In a hybrid forming section the stock jet isejected from a headbox slice onto a first forming fabric that istravelling in a horizontal plane in the machine direction over a seriesof dewatering boxes comprising a conventional open surface single fabricforming section. A second forming fabric is then brought into intimatecontact with the exposed upper sheet surface at the beginning of thehybrid two fabric forming section. The partially formed sheet and theundrained stock is sandwiched between two forming fabrics; drainage thenoccurs through both forming fabrics. The second forming fabric isseparated from the upper surface of the formed sheet at the end of thehybrid two fabric forming section and the sheet is conveyed to the presssection on the first forming fabric. This invention is concerned withthat portion of the hybrid two fabric forming section between the locusat which the first and second forming fabrics come together to sandwichthe stock between them and the locus at which the first and secondforming fabrics separate with the sheet continuing on the first formingfabric. Although the forming section described here includes a singlesecond forming fabric section this invention is not so limited. It iscommon to have more than one hybrid two fabric forming section, and tohave a second headbox delivering a second layer of stock onto the firstforming fabric ahead of the second hybrid two fabric forming section.

In a hybrid type forming section the two forming fabrics do not follow alinear path. The fabrics together pass over a sequence of rolls anddewatering boxes which are located on alternate sides of the two fabricsand thus define the sinuous path of the two fabrics. Each dewatering boxhas a curved surface, which carries a group of fabric support elements,such as blades, which are in contact with the machine sides of theforming fabrics. Each dewatering box may also be connected to a sourceof controlled vacuum. These curved surfaces cause the moving formingfabrics to follow the desired sinuous path. The application of acontrolled level of vacuum to the dewatering boxes has two effects: itpromotes the removal of water from the stock between the two movingforming fabrics, and it deflects the path of the two moving formingfabrics into the gaps between the fabric support elements. Thisdeflection of the two moving forming fabrics generates a positivepressure pulse within the stock layer sandwiched between them thatcreates fluid movement within the stock in the machine direction; thiscauses a shearing action within the stock which serves to break up fibreflocs.

The actual magnitude of each pressure pulse generated by the deflectionangle of the moving forming fabrics at the edges of each fabric supportelement has a significant impact on the quality of the final sheetproduced. The strength of the pressure pulse generated by each fabricsupport element should be chosen to match the stock conditions andproperties at that fabric support element. Hence, there exists a need tobe able to modify the strength and/or magnitude of the pressure pulsesas more water is drained from the stock and the incipient paper web isformed.

Poor control of the fabric deflection within the forming section hasbeen found to have an adverse effect on the formation process, whichwill in turn have a negative impact on the quality of the paper productbeing made.

The actual fabric deflection angle at the edge of each fabric supportelement in an operating twin fabric forming section has been found to becontrolled by several factors. These include:

1. the geometric layout of the physical components used in theconstruction of the forming zone; including the element-to-element pitchfor the fabric support elements, the machine direction width of thefabric support elements, and the radius of curvature of the surfaces towhich the fabric support elements are attached;

2. the level of vacuum applied to the dewatering boxes which controlsthe degree to which the moving forming fabrics are deflected into thegaps between the fabric support elements; and

3. the amount of machine direction tension applied to each of the twomoving forming fabrics.

As used herein, then following terms are to be taken to have thefollowing meanings:

(i) the term machine direction, or MD, refers to a direction generallyparallel to the direction of movement of the forming fabrics away from aheadbox slice;

(ii) the term “pitch” refers to the centre to centre spacing ofsuccessive fabric support elements in the machine direction; and

(iii) the terms “fabric support element” and “fabric support elements”refer:

either to moving surfaces such as rolls over which a forming fabricmoves in rolling contact,

or to static surfaces such as blades, foils or the like over which aforming fabric moves in sliding contact.

In the initial stages of sheet formation, when the level of vacuumapplied to the machine side of the forming fabric, and consequently tothe incipient paper web, is low, the predominant factors controllingforming fabric deflection are the geometry of the forming section andthe tension applied to both of the forming fabrics. Further, althoughthe tension applied to the two forming fabrics is usually the same, twodifferent tension levels can be used. The two tensions are set, withinthe overall pattern of adjustments, to obtain the desired level ofpressure pulses within the stock sandwiched between the two movingforming fabrics.

From the point at which the stock is first sandwiched between the twomoving forming fabrics until the point at which the two forming fabricsseparate, the consistency of the stock is continually increasing aswater is drained from the incipient paper web. At the same time as thestock consistency increases, there is also a corresponding decrease inindividual fiber mobility within the stock. These changes require astronger pressure pulse to provide beneficial fiber movement which willimprove the sheet properties in the incipient paper web. However, theincipient paper web eventually reaches a consistency at which no furtherbeneficial fiber movement can occur. From that point onwards until thetwo moving forming fabrics separate the pressure pulse strength must becontrolled by careful selection of the required vacuum level so thatdrainage continues, and by careful selection of the radius, fabricsupport element pitch and fabric support element width so that thepressure pulse strength is controlled to a level which will not act toimpair formation of the incipient paper web.

During the initial sheet forming period where beneficial fiber movementcan still occur, the need for a larger pressure pulse may increase at afaster rate than can be achieved by control of the vacuum level appliedto the forming fabrics alone. This is because the vacuum level must belimited to a value which does not cause excessive drainage which willboth reduce fiber mobility and set the sheet properties before thedesired formation benefits can be achieved. It is therefore essential toobtain a larger pressure pulse by causing a higher deflection of theforming fabrics at the edges of the fabric support elements by utilizinga wider pitch between them and/or by utilizing a higher radius ofcurvature in the structure to which the fabric contacting fabric supportelements are attached, and/or by utilizing opposed fabric supportelements, such as blades, located to increase fabric deflection into thegaps between the fabric support elements.

SUMMARY

It is thus apparent that there is a matrix of variables which must beconsidered in order to optimise the quality of the sheet product. Thepresent invention is based on the realization that the following factorsmust to be taken into account in the creation of an improved twin fabrichybrid type forming section for paper making machine:

(a) the pitch of the fabric support elements should decreaseprogressively in the machine direction;

(b) the level of vacuum applied to the forming fabrics through thedewatering boxes should increase in the machine direction;

(c) the two forming fabrics together with the stock sandwiched betweenthem should traverse at least four separate and distinct vacuum zoneswithin the forming section as they proceed in the machine direction;

(d) the level of vacuum applied to the last of the at least fourseparate and distinct vacuum zones must be higher than the level ofvacuum applied to the first of the separate and distinct vacuum zones;

(e) the level of vacuum applied to the at least four separate anddistinct vacuum zones must follow a preselected profile; and

(f) the dewatering boxes carrying the fabric support elements should bearranged so that the fabric support elements are located in analternating sequence on the machine sides of both of the formingfabrics.

Thus in a first broad embodiment this invention seeks to provide a twofabric hybrid type forming section for a paper making machine having afirst forming fabric and at least one second forming fabric, such that:

(i) each of the forming fabrics has a paper side and a machine side;

(ii) the forming fabrics move together in close proximity with eachother in the machine direction with a layer of stock sandwiched inbetween;

(iii) the forming fabrics are supported by a series of rolls and/or aseries of static fabric contacting fabric support elements over whichthe machine sides of each of the forming fabrics pass in slidingcontact, the fabric support elements being supported on a sequence ofdewatering boxes, the dewatering boxes each having a curved fabricsupport element supporting surface; and

(iv) the dewatering boxes provide separate drainage zones at least someof which are connected to a source of vacuum to provide separate vacuumzones,

wherein:

(a) the forming zone comprises that portion of the forming sectionbetween the locus at which the forming fabrics come together to sandwichthe stock between them and the locus at which the two forming fabricsseparate with the stock continuing on one of them;

(b) the dewatering boxes provide at least four separate and distinctvacuum zones within the forming section;

(c) either: the radii of curvature of the curved surfaces located overthose dewatering boxes which are connected to a source of vacuumsupporting the fabric supporting elements decreases progressively in themachine direction,

or: the radii of curvature of the curved surfaces located over thosedewatering boxes which are connected to a source of vacuum supportingthe fabric support elements decreases on successive support surfaces inthe machine direction;

(d) either: the pitch of the fabric support elements within each vacuumzone is constant, and the pitch of the fabric support elements onsuccessive vacuum zones decreases in the machine direction;

or: the pitch of successive fabric support elements within each vacuumzone decreases in the machine direction.

(e) the dewatering boxes supporting the fabric support elements areconstructed and arranged to locate the fabric support elements incontact with the machine sides of the first forming fabric and thesecond forming fabric in an alternating sequence in the machinedirection;

(f) on all of the dewatering boxes:

either: all of the fabric support elements are the same width in themachine direction;

or: all of the fabric support elements are not the same width in themachine direction.

Preferably, the fabric support element pitch within each vacuum zone isconstant, and the fabric support element pitch within successive vacuumzones decreases in the machine direction. Alternatively, the fabricsupport element pitch within each vacuum zone is not constant, and thefabric support element pitch within each successive vacuum zonedecreases in the machine direction.

Preferably, the radii of curvature of the curved surfaces supporting thefabric support elements on successive vacuum zones decreases in themachine direction. Alternatively, the radii of curvature of the curvedsurfaces supporting the fabric support elements on successive vacuumzones decrease progressively in the machine direction.

Preferably, each dewatering box provides at least one vacuum zone. Morepreferably, at least one dewatering box provides at least two vacuumzones. Most preferably all of the dewatering boxes provide more than onevacuum zone.

Preferably, the ratio of the width of the fabric support elements to thewidth of the gap between them varies from about 1:10 down to about1:0.5.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the attachedfigures in which:

FIG. 1 shows schematically a two fabric hybrid type forming sectionaccording to first embodiment of the invention;

FIG. 2 shows schematically in more detail the hybrid forming zone ofFIG. 1;

FIG. 3 shows schematically an alternative construction to FIG. 2; and

FIG. 4 shows schematically a further alternative construction to thatshown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIG. 1, a two fabric hybrid type forming section 1 isshown. The forming section 1 is arranged substantially horizontally; thearrow A indicates the horizontal direction.

In the forming section of this invention, the formation zone 60 wherethe sheet is formed on the first forming fabric 2 extends from thebreast roll 50 to the couch roll 57. A layer of stock 7 is ejected fromthe headbox slice 8 onto the first forming fabric 2. Within this zone 60the two fabric hybrid forming section extends from the locus where thefirst forming fabric 2 carrying the layer of stock 7 contacts the secondforming fabric 4 at lead-in box 53 sandwiching the stock 7 between them,to the locus of the turning roll 9 and transfer box 55 where the firstand second forming fabrics separate. The sheet continues towards thepress section on the first forming fabric 2. The two forming fabricsmove together through the hybrid forming section 1 so that the sheetmoves in the machine direction as indicated by arrow A.

Although the hybrid forming section 1 shown in FIG. 1 includes a singleso-called “top wire” forming unit 61, located on the first formingfabric 2, other arrangements are possible. For example more than one ownheadbox delivering additional stock onto the first forming fabric 2.Each additional unit 61 can also be provided with its own headboxdelivering additional stock onto the first forming fabric 2.

In the operation of the formation zone 60, a jet of stock is ejectedfrom the headbox slice 8 to provide a layer 7 of very aqueous stock onthe open surface portion 2A of the first forming fabric 2. The firstforming fabric 2 and the stock layer 7 move together in the machinedirection shown by arrow A, over in sequence a forming board 51, and aseries of dewatering boxes and other sundry dewatering devices indicatedgenerally as 52. The first forming fabric 2 carrying the stock layer 7then enters the top wire unit 61 of the hybrid forming section 1. Thesecond forming fabric 4 is brought into contact with the stock layer 7at this point, so that it becomes sandwiched between the first andsecond forming fabrics 2 and 4 (see FIG. 2 for more details). The firstforming fabric 2 and the second forming fabric 4, with the stock layer 7sandwiched between them, then pass with their respective machine sidesin contact with a sequence of units. These are: a lead-in dewatering box53, a multi-chambered dewatering box 10, an opposed fabric supportelement unit 54 and a transfer box 55. The multi-chambered dewateringbox 10 is located with its fabric support elements in contact with themachine side of the second forming fabric only (see FIGS. 2, 3 and 4).At the end of the unit 61 the second forming fabric 4 wraps around aturning roll 9 and is thereby taken out of contact with the stock layer7. The stock layer 7 carried by the first forming fabric 2 then passesover further dewatering boxes 56 and finally is transferred after thecouch roll 57 at the end of the forming section 61 to the press section(not shown) for further processing.

FIG. 2 shows a more detailed schematic view of the lower part of the twofabric hybrid forming section 1 shown in FIG. 1. In FIG. 2 the secondforming fabric 4 partially wraps around the forming roll 3 with theresult that the stock 7, which is conveyed in the machine direction asindicated by the arrow A, becomes sandwiched between the first formingfabric 2 and the second forming fabric 4. The two forming fabrics 2 and4 with the stock layer 7 sandwiched between them then pass over severaldewatering devices. The machine side of the first forming fabric 2passes in sliding contact over the lead-in dewatering box 53, an opposedfabric support element box 54 and a transfer box 55. At the same time,the machine side of the second forming fabric 4 passes in slidingcontact with the opposed fabric support elements 73 located on themulti-chambered dewatering unit 10. Box 54 is optional, and the supportelements 71 need not all be in contact with the machine side of thefabric 2. The two forming fabrics 2 and 4 thus pass together in sequencepast these four dewatering units in the sequence box 53, unit 54, unit10 and box 55. After box 55 the second forming fabric 4 wraps around theturning roll 9 and is carried away out of contact with the stock 7. Thestock 7 is carried by the first forming fabric 2 towards the presssection (not shown).

In FIG. 2, dewatering box 53, which is referred to as a lead-in box, asshown is provided with two vacuum chambers 63, 64. Box 55, which isreferred to as a transfer box, which ensures the transfer of the stock 7from the second forming fabric 4 to the first forming fabric 2, as shownis provided with a single vacuum chamber. Either or both of thesedewatering boxes 53 and 55 may be internally divided to provide two, ormore, separate vacuum chambers each of which is connected to a separatecontrolled vacuum supply (not shown). A further embodiment is shown inFIG. 4, in which Box 53 comprises a single vacuum chamber and Box 55comprises two vacuum chambers 101, 102.

In Box 53, forming fabric support elements 70 are mounted on thecontinuously curved fabric support element supporting surface 90. Box 54is an opposed fabric support element unit, which is a gravity drainagebox. Water removed from the machine side surface of the first formingfabric 2 drops into the box 54, and is removed therefrom. The box 54includes fabric support elements 71, which are mounted on the surface91. As this box 54 is on the outside of the convex curve of the twofabrics 2, 4, formed by the box 10, the fabric support elements 71 canbe mounted on flexible, adjustable mountings such as those disclosed byMcPherson in U.S. Pat. No. 6,361,657. Box 55 is provided with aplurality of fabric support elements 72 supported by the continuouslycurved surface 96.

FIG. 2 also shows a multi-chambered dewatering unit 10. As shown, unit10 includes four distinct vacuum zones 80, 81, 82 and 83, each of whichis provided with a separate controlled vacuum supply (not shown).Located beneath each of the separate vacuum zones 80, 81, and 82 is aset of fabric support elements, as at 73. The fabric support elements 73are supported on the curved surfaces 92, 93 and 94.

There are several possibilities for the radii of curvature of the threesurfaces 92, 93 and 94.

(i) The three radii of curvature can be the same, so that all threesurfaces 92, 93 and 94 together form a single constant radius curve.

(ii) At least one of the three radii can be different, or all three canbe different. If this arrangement is adopted, then the radius ofcurvature of each of the surfaces 92, 93 and 94 must decrease in themachine direction, so that the radius of curvature of the surface 94 isalways the smallest of the three.

It also apparent from FIG. 2 that the pitch of the fabric supportelements 73 on the multi-chambered dewatering unit 10 is not constant.The pitch decreases in the machine direction.

In FIG. 2, fabric support element 74 which is the first element of theset 73, is located on the upstream side of zone 80 towards the headboxslice and is a so-called autoslice blade, also known as a skimmer blade.When in use, the autoslice blade 74 skims excess water from the machineside of the second forming fabric 4 as it passes in the machinedirection in sliding contact with the element 74.

FIG. 3 is similar to FIG. 2, with the exception that on box 53 theradius of curvature of the curved fabric support element supportingsurface 90 is not constant. The surface 90 is broken into successiveportions having radii of curvature R₁, R₂ and R₃. The radius ofcurvature for each portion decreases in the machine direction, so thatR₁ is the largest radius of curvature. By decreasing the radius ofcurvature of the supporting surface 90 for the fabric support elements70 located on the lead-in box 53 so as to increase sequentially theamount of wrap of the first and second forming fabrics 2,4 the stock 7is subjected to increasingly stronger pressure pulses, which induceshearing actions within the stock 7, at each edge of the fabric supportelements 70 as the forming fabrics 2,4 pass over them in the machinedirection. This feature is also shown in each of the dewatering boxes53, 54, 10 and 55.

FIG. 4 is also similar to FIG. 2 except that the individual or discretefabric support elements 70 of the lead-in box 53 are replaced by thecontinuous curved surface 100 mounted on support surface 90, asdescribed by Buchanan et al. in US 2003/017438. In addition, thetransfer box 55 has been internally portioned to provide two separatevacuum zones 101 and 102, each of which is provided with its owncontrolled vacuum supply (not shown).

In the drawings the fabric support elements are all shown schematicallyto have the same width in the machine direction. In practise, the fabricsupport element width may not be the same for all of the dewateringboxes. Some dewatering boxes may require a different width fabricsupport element just to accommodate the volume of white water which isbeing drained from the forming fabrics at that location. It is alsopossible that a different width fabric support element may be requiredin order to obtain the desired level of pressure pulse within the stockat a given location. Experience shows that the ratio of the machinedirection width of fabric support elements to the width of the gapbetween them should be from about 1:10 to about 1:0.5.

In the drawings dewatering boxes are shown which have more than onechamber to each of which a controlled level of vacuum is applied. If thevacuum levels in adjacent chambers or dewatering boxes are not the same,it is desirable that the surface curvatures, and possibly also thecorresponding fabric support element pitch, also should not be the same.Furthermore experience shows that it is desirable that the vacuum levelin a sequence of dewatering boxes or chambers should increase relativelysmoothly in the machine direction. Although the vacuum level can remainconstant in two adjacent dewatering boxes or chambers it should notdecrease in the machine direction, and furthermore spikes of radicallydifferent pressure should be avoided. In other words, all of thevariables do not necessarily change smoothly in a step wise fashion;adjacent zones can have the same values for at least some of thevariables.

1. A two fabric hybrid type forming section for a paper making machinehaving a first forming fabric and at least one second forming fabric,such that: (i) each of the forming fabrics has a paper side and amachine side; (ii) the first forming fabric receives a layer of stock atan impingement point in a first open surface region, and thereafterpasses in sequence through the first open surface region, a centralforming zone and a second open surface region; (iii) the second formingfabric passes through the central forming zone such that the paper sideof the second forming fabric faces the paper side of the first formingfabric, and the two forming fabrics move together in the machinedirection with a layer of stock sandwiched in between; (iv) the formingfabrics are supported by a series of fabric support elements, chosenfrom the group consisting of rolls, static fabric support elements andboth rolls and static fabric support elements, over which the machinesides of each of the forming fabrics pass in sliding contact, the fabricsupport elements being supported on a sequence of dewatering boxes, thedewatering boxes having a curved fabric support element supportingsurface; and (v) the dewatering boxes provide separate drainage zonesand at least some of the dewatering boxes are connected to a source ofvacuum to provide separate vacuum zones, wherein: (a) the dewateringboxes provide at least four separate and distinct vacuum zones withinthe central forming zone; (b) the radii of curvature of the curvedsurfaces supporting the fabric support elements decrease progressivelyin the machine direction; (c) a pitch of the fabric support elementsdecreases progressively in the machine direction; and (d) the dewateringboxes supporting the fabric support elements are selectively located onthe machine sides of the first forming fabric and the second formingfabric in opposed and at least partially offset sequence in the machinedirection.
 2. A forming section according to claim 1, wherein the pitchof successive fabric support elements within at least one vacuum zonedecreases in the machine direction.
 3. A forming section according toclaim 1, wherein the fabric support elements have a width in the machinedirection which decreases progressively in the machine direction.
 4. Aforming section according to claim 1, wherein the fabric supportelements within at least one vacuum zone have a width in the machinedirection which decreases progressively in the machine direction.
 5. Aforming section according to claim 1, wherein each dewatering boxprovides at least one vacuum zone.
 6. A forming section according toclaim 5, wherein at least one dewatering box provides at least twovacuum zones.
 7. A forming section according to claim 6, wherein eachdewatering box provides at least two vacuum zones.
 8. A forming sectionaccording to claim 6, wherein in each dewatering box which provides atleast two vacuum zones, the fabric support element pitch withinsuccessive vacuum zones provided by the dewatering box decreases in themachine direction.
 9. A forming section according to claim 7, wherein ineach dewatering box which provides at least two vacuum zones, the fabricsupport element pitch within successive vacuum zones provided by thedewatering box decreases in the machine direction.
 10. A forming sectionaccording to claim 6, wherein in each dewatering box which provides atleast two vacuum zones, the radii of curvature of the curved surfacessupporting the fabric support elements within successive vacuum zonesprovided by the dewatering box decreases in the machine direction.
 11. Aforming section according to claim 7, wherein in each dewatering boxwhich provides at least two vacuum zones, the radii of curvature of thecurved surfaces supporting the fabric support elements within successivevacuum zones provided by the dewatering box decreases in the machinedirection.
 12. A forming section according to claim 1, wherein the ratioof the width of the fabric support elements to the width of the gapbetween them varies from about 1:10 to about 1:0.5.
 13. A formingsection according to claim 1, wherein the forming section furtherincludes a turning roll which is provided with vacuum assisted drainage.14. A forming section according to claim 1, wherein the forming sectionfurther includes a turning roll without vacuum assisted drainage.